An image sensor captures images using light-sensitive photosensitive areas that convert incident light into electrical signals. Image sensors are generally classified as either front-illuminated image sensors or back-illuminated image sensors. FIG. 1 is a simplified illustration of a front-illuminated image sensor in accordance with the prior art. Image sensor 100 includes pixels 102, 104, 106 formed within a semiconductor layer 108 and an interconnect layer 110. Photosensitive areas 112, 114, 116 are formed in semiconductor layer 108. Conductive interconnects 118, 120, 122, such as gates and connectors, are formed in interconnect layer 110.
Unfortunately, the positioning of conductive interconnects 118, 120, 122, and various other features associated with interconnect layer 110, over photosensitive areas 112, 114, 116 adversely impacts the fill factor and quantum efficiency of image sensor 100. This is because light 124 from a subject scene must pass through interconnect layer 110 before it is detected by photosensitive areas 112, 114, 116.
A back-illuminated image sensor addresses these fill factor and quantum efficiency issues by constructing the image sensor such that light from a subject scene is incident on a backside of the semiconductor layer 108. The “frontside” 126 of semiconductor layer 108 is conventionally known as the side of semiconductor layer 108 that abuts interconnect layer 110, while the “backside” 128 is the side of semiconductor layer 108 that opposes frontside 126. FIG. 2 is a simplified illustration of a back-illuminated image sensor 200 in accordance with the prior art. Interconnect layer 110 is positioned between support substrate 202 and semiconductor layer 108. This allows light 124 to strike the backside 128 of semiconductor layer 108, where it is detected by photosensitive areas 112, 114, 116. Light detection by photosensitive areas 112, 114, 116 is no longer impacted by the metallization level interconnects and other features of interconnect layer 110.
One of the other features associated with interconnect layer 110 are bond pads. Bond pads are used to transmit signals to, and receive signals from, various circuits and components in an integrated electrical component, such as an image sensor. A wire affixed to a bond pad is electrically connected to one or more circuits or components in the image sensor. FIG. 3 is a graphical illustration of a wire affixed to a bond pad in a back-illuminated image sensor 300 in accordance with the prior art. Opening 302 is formed through semiconductor layer 304 to expose a bond pad 306 in interconnect layer 308. If wire 310 contacts semiconductor layer 304 when affixed to bond pad 306, such as at area 312, wire 310 is electrically connected to semiconductor layer 304 and produces an electrical short to the image sensor. The electrical short renders the image sensor unusable. Electrical shorts like this are not an issue for front-illuminated image sensors because the bond pads are in the interconnect layer which is positioned above the semiconductor layer. The bond wire is not able to contact the semiconductor layer.
Electrical shorts can also occur at wafer level testing when a tester accidentally touches semiconductor layer 304 with a probe pin. The tester may report the die is faulty when there is no actual problem with the image sensor.
Failures at both package and wafer testing reduce yield, and hence increase costs. Several isolation techniques have been used to prevent electrical shorts from damaging the image sensors. FIG. 4 is a graphical depiction of a first isolation technique in back-illuminated image sensors in accordance with the prior art. A conformal insulating material 400 is deposited over the image sensor and lines the sidewalls of opening 402. The insulating material 400 electrically isolates semiconductor layer 404 from a wire (not shown). The conformal insulating material 400, however, narrows the width 406 of opening 402 such that in some situations, a wire can not be affixed to bond pad 408 because the wire is larger than width 406.
FIG. 5 is a graphical illustration of a second isolation technique in back-illuminated image sensors in accordance with the prior art. Deep trench isolation regions 500 are formed with dielectric material and extend from the frontside 502 of semiconductor layer 504 to the backside 506 of semiconductor layer 504. If wire 508 contacts semiconductor layer 504 when affixed to bond pad 510, such as at area 512, deep trench isolation regions 500 electrically isolate semiconductor layer 504 and prevent wire 508 from producing an electrical short to the image sensor. Unfortunately, the fabrication of deep trench isolation regions is a complex procedure that requires many processing steps. This complexity increases the cost to produce image sensors that utilize this isolation technique.